1. Field of the Invention
The present invention relates to a backlight unit and a Liquid-Crystal Display (LCD) device and more particularly, to a backlight unit with point-shaped light sources such as Light-Emitting Diodes (LEDs), which is preferably applicable to LCD devices, and a LCD device using the backlight unit.
2. Description of the Related Art
In recent years, the LCD device has been extensively used as high-resolution displays. Generally, the LCD device comprises a substrate on which switching elements such as Thin-Film Transistors (TFTs) are arranged, which will be termed a “TFT substrate” below; an opposite substrate on which a color filter, a black matrix or the like are formed; and a liquid-crystal layer intervening between the TFT substrate and the opposite substrate. By changing the alignment direction of the liquid-crystal molecules in the liquid-crystal layer with the electric field generated between the pixel electrodes on the TFT substrate and the common electrode on the opposite substrate or between the common and pixel electrodes on the TFT substrate, the amount of transmitting light in the respective pixels is controlled to display images on the screen of the LCD device. The assembly comprising the TFT substrate, the opposite substrate, and the intervening liquid-crystal layer between these two substrates is termed the liquid-crystal display panel or LCD panel.
With the transmissive and semi-transmissive type LCD devices, a backlight unit is incorporated as a planer light source, because the liquid crystal per se does not emit light. The light emitted from one surface of the backlight unit, i.e., backlight, is designed in such a way as to be irradiated to the LCD panel. The backlight unit is divided into two types, the “direct type” and the “edge-light type”. With the “direct type” backlight unit, linear or point-shaped light sources are arranged directly underneath the LCD panel with a predetermined layout. On the other hand, with the “edge-light type”, a linear light source or sources or point-shaped light sources is/are arranged along an edge or edges of a light guide plate disposed right under the LCD panel.
With the conventional backlight units, a cold-cathode fluorescent lamp has been popularly used as a linear light source. However, a cold-cathode fluorescent lamp contains mercury (Hg) and thus, there is a problem that it gives bad effects to the environment largely. Moreover, since a cold-cathode fluorescent lamp necessitates high voltage for emitting light, there is another problem that it is likely to generate noises. Accordingly, recently, there has been a growth in the use of LEDs as a point-shaped light source.
Where LEDs are used as the light source instead of cold-cathode fluorescent lamps, obtainable luminance by a white LED for emitting white light or by a set of three LEDs for respectively emitting red, green, and blue monochromatic light is insufficient. Therefore, it is popular that a plurality of white LEDs or a set of plural red, green, and blue LEDs is linearly arranged to form a linear light source. Such the combination of LEDs as explained here is termed a “LED unit” below. This is because there is an advantage that the LED unit can be treated in designing in a similar way to the cold-cathode fluorescent lamp and therefore, the know-how and the like obtained for the cold-cathode fluorescent lamp may be applied to the LED unit.
With the direct type backlight unit, however, the obtainable luminance on the diffusing plate provided for diffusing the output light emitted from the light source varies dependent upon the location. Specifically, the obtainable luminance on the diffusing plate in the region immediately above the LED unit is higher than that in the remaining region. Thus, the luminance distribution on the display screen is likely to be uneven. Since this leads to unevenness in color and/or luminance, there is the need to adjust the said luminance distribution.
Moreover, with the LED unit formed by combining LEDs each emitting red, green, or blue monochromatic light, there is the need to mix the red, green, and blue light to generate white light. (Such the need is trivial for the LED unit formed by aligning white LEDs alone.) Therefore, it is essential to increase the distance between the LEDs and the diffusing plate to some extent. This means that the backlight unit and the LCD device incorporating the same will be large-sized.
To solve these two problems, i.e., “the non-uniformity of the luminance distribution” and “the enlargement in size”, conventionally, various improvements have been made. Examples of these improvements are shown in FIGS. 1A and 1B. Both of the prior-art backlight units shown in FIGS. 1A and 1B are of the direct type, which are disclosed in the patent document 1 (Japanese Non-Examined Patent Publication No. 2004-311353 published in 2004). (See claim 1, paragraphs 0010-0026 and 0046-0049, and FIGS. 1-3 and 11.)
With the prior-art backlight unit of FIG. 1A, plate-shaped reflectors are respectively formed on the inner bottom face 101a and the inner side face of a housing 101. The reflector formed on the bottom face 101a is termed the first reflector 102. The opening 101b of the housing 101, which is opposite to the bottom face 101a, is closed or blocked by a diffusion plate 103 for transmitting and diffusing the light.
As the point-shaped light sources 104, a plurality of LEDs each emitting red (R), green (G), or blue (B) monochromatic light is combined together. The LEDs 104 are mounted on each of the point-shaped light source substrates 105 along its longitudinal direction, which is perpendicular to the paper. Here, the count of the substrates 105 is three, which are aligned at predetermined intervals. The arrangement of the LEDs 104 mounted on each of the substrates 105 is made by repetition of a specific order, for example, G, B, G, R, G, and B. Each substrate 105 is fixed on the outside of the bottom face 101a of the housing 101, and the LEDs 104 mounted on the said substrate 105 are exposed from the bottom face 101a to the inside of the housing 101 through its bottom wall.
Rectangular plate-shaped second reflectors 106, the count of which is three, are provided in the housing 101 in such a way as to be superposed on the respective substrates 105. Each of the second reflectors 106 has a reflective surface 106a opposite to the corresponding first reflector 102. The reverse of the reflective surface 106a is a regular reflection surface 106b. The second reflectors 106 are fixed on the inner side face of the housing 101 in such a way as to be approximately parallel to the first reflectors 102. A gap is formed between the side face of the housing 101 and the second reflector 106 adjacent thereto, and another gap is formed between the adjoining second reflectors 106. Thus, the light emitted from the point-shaped light sources 104 can reach the side of the diffusing plate 103 by way of these gaps.
With the prior-art backlight unit having the above-described configuration of FIG. 1A, the R, G, and B monochromatic light beams emitted from the LEDs or point-shaped light sources 104 are directly reflected by the first reflector 102 and then, reflected by the reflective surfaces 106a of the second reflectors 106. Alternately, these light beams are reflected by the reflective surfaces 106a of the second reflectors 106, and reflected by the first reflector 102 and thereafter, reflected again by the reflective surfaces 106a. In this way, these monochromatic light beams are repeatedly reflected and propagated between the first reflector 102 and the second reflectors 106 and as a result, they are mixed together and uniformized to white light. The white light thus generated will reach the diffusing plate 103 by way of the gaps between the side face of the housing 101 and the second reflectors 106 and the gaps between the adjoining second reflectors 106.
The light incident on the diffusing plate 103 is divided into a component that penetrates through the inside of the diffusing plate 103 and another component that is reflected by the particles in the diffusing plate 103 toward the side of the point-shaped light sources 104. The reflected component of the said light is reflected by the first reflector 102 or the regular reflection surfaces 106b of the second reflectors 106 and is incident again on the plate 103. The outgoing light from the diffusing plate 103 will radiate from its surface in all directions uniformly.
As explained above, the R, G, and B monochromatic light emitted from the LEDs 104 are repeatedly reflected and propagated in the space between the first reflector 102 and the second reflectors 106 and therefore, sufficient distances for mixture to white light are obtained. As a result, the color unevenness of the LCD device can be prevented from occurring without enlargement in size.
In addition, conventionally, the luminance in the region immediately above the LEDs (i.e., the point-shaped light sources) 104 is higher than that in the remaining or surrounding region thereof and thus, the luminance distribution on the display screen is likely to be uneven. Unlike this, with the prior-art backlight unit of FIG. 1A, because the second reflectors 106 are provided between the LEDs 104 and the first reflector 102, such the luminance unevenness can be suppressed.
The prior-art backlight unit shown in FIG. 1B has the same configuration as the prior-art backlight unit shown in FIG. 1A except that patterned light-shielding layers 110a are selectively printed on a surface (i.e., the inner surface in FIG. 1B) of the diffusing plate 103. Each of the light-shielding layers 110a has a diffuse reflection function of incident light. Each of the light-shielding layers 110a is located in the area to which the light is irradiated through the gaps between the side face of the housing 101 and the second reflector 106 adjacent thereto and the gaps formed between the adjoining second reflectors 106. The light-shielding layers 110a are formed by vacuum evaporation or silk printing of aluminum (Al). The size, density and gradation of the ink dots and/or the deposited patterns constituting the layers 110a are adjusted to realize uniform luminance distribution.
With the prior-art backlight unit of FIG. 1B, the light passing through the gaps between the side face of the housing 101 and the second reflector 106 adjacent thereto and the gaps between the adjoining second reflectors 106 reaches the light-shielding layers 110a and is diffuse-reflected by the layers 110a and then, further diffused in the housing 101. Thus, the luminance unevenness and color unevenness are more likely to be suppressed than the prior-art backlight unit of FIG. 1A.
Moreover, although not shown, still another prior-art direct type backlight unit is disclosed in the patent document 2 (Japanese Non-Examined Patent Publication No. 2005-117023 published in 2005). (See claim 1, paragraphs 0129-0131 and 0143-0147, and FIGS. 17 and 24.) This backlight unit comprises a similar structure to the patterned light-shielding layers 110a of the prior-art backlight unit of FIG. 1B.
With the structure shown in FIGS. 17 and 24 of the patent document 2, a plurality of LED units are arranged at intervals on the inner bottom surface of a housing. Each of the LED units comprises LEDs aligned regularly. A diffusing plate is fixed at the mouth of the housing located on the opposite side to the bottom surface. A diffusing light guide plate is provided between the bottom surface and the diffusing plate. Patterned light-controlling dots are formed on a surface of the diffusing light guide plate. Each of the light-controlling dots is placed in a one-on-one relationship with an opposing one of the LEDs. These dots are formed by printing with ink.
Each of the light-controlling dots reflects the incident light due to the reflection property of the ink. At the same time, each of the dots diffuse-reflects the incident light efficiently due to the shielding property of the light-shielding agent added to the ink and the diffusion property of the diffusing agent added thereto. Accordingly, generation of high-luminance regions termed the lamp images is prevented, in other words, luminance unevenness is suppressed, which results in equalized luminance.
Moreover, because of the light-controlling dots, the light penetrating through the diffusing light guide plate exhibits high color mixing property. Therefore, the color unevenness of the resultant light is suppressed significantly.
With any of the above-described prior-art direct type backlight units, luminance unevenness and color unevenness can be suppressed without enlargement in size. However, as long as a set of LEDs (i.e., point-shaped light sources) emitting red, green, and blue monochromatic light is used in combination, it is inevitable that color unevenness is left on the display screen in accordance with the placement order of the LEDs in the set. For example, if a red LED emitting red light is placed at one end of the LED unit, color mixture is difficult to occur with respect to the red LED. This is because a green or blue LED is not placed adjacent to the said red LED on one side thereof. Therefore, the corresponding position on the display screen to the said red LED contains some redness compared with the other positions.
The above problem of color unevenness for the direct type backlight unit will occur in the edge-light type backlight unit. In particular, this phenomenon is more likely to occur if the edge-light type backlight unit comprises a single LED unit formed by a set of red, green and blue LEDs aligned in a single direction. This is because the light emitted from the respective LEDs of plural LED units is unable to be mixed together.
Furthermore, since all the above-described prior-art backlight units are of the direct type, the patterned light-shielding layers and the patterned light-controlling dots used therein are not easily applied to the edge-light type backlight units.